Cancer remains a major cause of death in the 21st century. Consequently, considerable drug research and development effort is currently placed on the discovery of therapeutics that may provide life extending or curative options to cancer sufferers.
While there are many different varieties of cancer, each exhibiting a different array of genetic and growth properties, a common denominator among many solid cancer types is the ability to metastasise. Until the occurrence of metastasis, tumors are confined to one area of the body and may be controlled through surgical intervention and/or radiotherapy. However, metastasis causes cancer cells to spread to disparate parts of the body and while surgical intervention may remove the primary tumor lesion, removal of all metastatic lesions is very difficult to manage.
Tumor metastasis is a multistage process, involving the breakdown of extracellular matrix, invasion of local tissue parenchyma, intravasation into regional blood vessels and lymphatics, survival in the circulation and finally extravasation, survival and growth in secondary tissue sites (Front. Biosci. (Elite Ed). 2012; 4: 1888-1897).
Metastasis may occur through blood vessels or lymphatic vessels. Lymphatic vessels differ from blood vessels in several ways. Large collecting lymphatic vessels contain vascular smooth muscle cells in their wall, as well as valves, which prevent the backflow of lymph. However, lymphatic capillaries, unlike typical blood capillaries, lack pericytes and continuous basal lamina and contain large inter-endothelial valve-like openings (J. Theor. Med. 2003; 5: 59-66). Due to their greater permeability, lymphatic capillaries are more effective than blood capillaries in allowing tumor cells to pass. Experimental evidence demonstrates that lymphangiogenesis (the formation of new lymphatic vessels) within a growing tumor lesion promotes metastasis through lymphatic vessels. The control of lymphangiogenesis presents an attractive therapeutic strategy for preventing lymph node metastasis (J. Clin. Onc. 2007; 25: 4298-4307).
The lymphatic system is comprised of capillaries and larger collecting vessels continuously lined by endothelial cells which return extravasated fluid and macromolecules from the interstitial space back to the blood circulation. Metastasis to regional lymph nodes via lymphatic vessels is a tumor progression process that is common to many cancer types. The extent of lymph node involvement is a major determinant for the staging of many types of cancer and is an important prognostic factor that is used as the basis for surgical and radiation treatment intervention of the affected lymph nodes.
Molecular signalling through binding of the growth factors VEGFC or VEGFD to their membrane receptor VEGFR3 has been shown to play a central role in the process of lymphangiogenesis (Brit. J. Cancer 2006; 94: 1355-1360). Stimulation of the VEGFR3 receptor occurs through the phosphorylation of its intracellular region and triggers a downstream signalling cascade that drives lymphatic endothelial cell proliferation, migration and differentiation leading to formation of lymphatic vessels (Exp. Cell Res. 2006; 312: 575-583). Increased expression of VEGFC or VEGFD has been shown to promote tumor associated lymphangiogenesis enabling lymphatic-mediated metastasis to regional lymph nodes. These observations have been reported for several different tumor types, including colorectal (Oncol. Rep. 2009; 22: 1093-1100) lung (Ann. Oncol. 2010; 21: 223-231), gastric (Surgery 2009; 146: 896-905), kidney (Oncol. Rep. 2008; 20: 721-725) prostate (Clin. Cancer Res. 2004; 10: 5137-5144) and ovarian (Cancer 2004; 101: 1364-1374). Blockade of VEGFC, VEGFD/VEGFR3 mediated signalling has been shown to inhibit lymphangiogenesis and suppress lymph node metastasis in several tumor experimental models in rodents (Ann. N.Y. Acad. Sci. 2008; 113: 225-234; Int. J. Cancer 2009; 125: 2747-2756).
VEGFR3 is a transmembrane tyrosine kinase receptor that is broadly expressed in endothelial cells during embryogenesis (Biochem. J. 2011; 437: 169-183). In the latter stages of development VEGFR3 expression becomes restricted to developing lymphatic vessels. In adults, VEGFR3 expression is primarily restricted to lymphatic endothelium and a subset of CD34+ hematopoietic cells. In addition, fenestrated capillaries and veins in certain endocrine organs, as well as monocytes, macrophages and some dendritic cells (DCs), continue to express VEGFR3 in adults. Disruption of the VEGFR3 gene in mouse embryos results in the failure of vascular network formation and death after embryonic day 9.5 (Biochem. J. 2011; 437: 169-183). This observation demonstrates that VEGFR3 plays an essential role in the development of embryonic vasculature. In cancer, VEGFR3 is overexpressed in lymphatic sinuses in metastatic lymph nodes and in lymphangiomas. Furthermore, in many instances cancer cells themselves express VEGFR3. VEGFR3 expressing cancer cells have been shown to be dependent on VEGFR3/VEGFC signalling for their proliferation (Eur. J. Canc. 2011; 47: 2353-2363).
Based on the foregoing, it is apparent that inhibition of VEGFR3 signalling has strong potential as therapeutic strategy for mammalian subjects that have been diagnosed with a disease characterised by proliferation of endothelial cells that express this receptor. In the case of cancer, targeting VEGFR3 is likely to result in therapeutic benefit through suppression of lymphatic metastasis and suppression of growth in cancer cells that express VEGFR3.
Interestingly, and perhaps importantly from the view point of target selection within the VEGFR3 axis, in mice in which both the VEGFC and the VEGFD genes have been homozygously deleted, the blood vasculature develops normally, unlike the embryonic cardiovascular phenotype of VEGFR3 homozygous knockout mice: i.e. deletion of these two ligands is not the same as deletion of the receptor (Mol. Cell. Biol. 2008; 28: 4843-4850). These data raise the possibility that another ligand for VEGFR3 exists or that VEGFR3 may be able to act by an as-yet-unknown manner independent of its ligands VEGFC and VEGFD. The foregoing suggest that targeting VEGFR3 is more advantageous to blocking VEGFC/D-VEGFR3 signalling compared to targeting either VEGFC or VEGFD alone.
Whilst there are a number of studies reported involving tyrosine kinase inhibitors with various levels of VEGFR3 activity and selectivity (Nat. Rev. Drug Discov. 2006; 5: 835-844; Mol. Cancer Ther. 2007; 6: 2012-2021; Cancer Res. 2009; 69: 8009-8016; Mol. Cancer Ther. 2012; 11: 1637-1649) these studies have some limitations, resulting in part at least from inhibition at other tyrosine kinases.
Nonetheless, collectively these studies strengthen the conclusion that inhibition of VEGFR3 suppresses or reduces lymphangiogenesis and/or lymphogenic metastasis.
Accordingly, compounds that selectively inhibit VEGFR3 would be useful for the treatment of proliferative diseases, such as cancer.
As described above, VEGFR3 plays an important role in the control of lymphangiogenesis. Accordingly, inhibitors of VEGFR3 may have utility in the treatment of diseases other than cancer where control/inhibition of lymphangiogenesis has a therapeutic benefit. The lymphatic system plays a major role in chronic inflammatory diseases and in transplant rejection. Inhibition of lymphangiogenesis through suppression of VEGFR3 function may provide a viable therapeutic strategy in these conditions.
For example, preclinical studies have demonstrated that the expression of VEGFR3 in the cornea and ocular surface is modified during corneal neovascularisation and that VEGFR3 mediates corneal dendritic cell migration to lymph nodes and induction of immunity to corneal transplant. High-risk corneal transplantation, where grafting is performed on inflamed and highly vascularized host beds, has a very poor success rate, with rejection rates as high as 90% (J. Leukoc Biol. 2003; 74: 172-178). In preclinical models, treatment with a VEGFR3 antibody leads to significant suppression of corneal graft rejection (Nat. Med. 2004; 10: 813-815).
Choroidal neovascularization (CNV), the creation of new blood vessels in the choroid layer of the eye, leads to chronic inflammation which is implicated in the pathogenesis of age related macular degeneration (AMD) and is driven by factors which include uncontrolled expression of the vascular endothelial growth factor (VEGF) family members VEGFA and VEGFC (J. Cell. Physiol. 2012; 227(1): 116-26). Treatments for AMD have been developed that target VEGFA, for example the anti-VEGFA antibodies ranibizumab and bevacizumab and the anti-VEGF aptamer pegaptanib, but to date no treatments have been clinically evaluated that mediate effects through modulation of VEGFC and its cognate receptor VEGFR3.
Accordingly, compounds that inhibit VEGFR3 may be useful for the prevention and/or treatment of eye diseases, for example corneal graft rejection and age related macular degeneration.
Furthermore, there is increasing evidence that lymphatic vessels have an active role in chronic inflammation of the skin. Lymphatic endothelial cell proliferation and lymphatic hyperplasia have been described in chronic skin inflammation in mice and have been reported for skin lesions in psoriasis patients (Blood 2004; 104: 1048-1057).
Accordingly, compounds that inhibit VEGFR3 may be useful for the prevention and/or treatment of skin inflammations, such as skin lesions in patients with psoriasis.
Lymphangiogenesis has also been found to be associated with kidney transplant rejection. VEGFC producing macrophages induce formation of new lymphatics which induce and support the maintenance of an alloreactive immune response in renal transplants (Nat. Med. 2006; 12: 230-234).
Accordingly, compounds that inhibit VEGFR3 may be useful for the prevention and/or treatment of rejection in renal transplantation.
Co-pending application WO2012/110773 discloses compounds which inhibit FAK and VEGFR3.